Orthogonally Functionalized Donor/Acceptor Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: An Approach to Introduce Panchromaticity and Control the Charge Recombination

Organic dyes possessing conjugated π-framework forms closely packed monolayers on photoanode in dye-sensitized solar cell (DSSC), because of the limitation to control the orientation and the extend of intermolecular π–π interaction, self-aggregation of dyes leads to reduced cell performance. In this report, a series of homodimeric ( D 1 -D 1 and D 2 -D 2 ) and heterodimeric ( D 1 -D 2 and D 2 -D 4 ) donor/acceptor (D/A) dyes containing spiroBiProDOT π-spacer were designed and synthesized by utilizing Pd-catalyzed direct arylation reaction and correlates the device performance with monomeric dyes ( D 1 and D 2 ). Both the thiophenes (π-spacer) of spiroBiProDOT were functionalized with same or different donor groups which led to homodimeric and heterodimeric chromophores in a single sensitizer. The homodimeric spiro-dye D 1 -D 1 showed higher power conversion efficiency (PCE), of 7.6% with a V oc and J sc of 0.672 V and 16.16 mA/cm 2, respectively. On the other hand, the monomeric D 1 exhibited a PCE of 3.2% ( V oc of 0.64 V and J sc of 7.2 mA/cm 2 ), which is lower by 2.4 fold compared to dimeric analogue. The spiro-unit provides flexibility between the incorporated chromophores to orient on TiO 2 due to four sp 3 -centers, which arrest the molecular motions after chemisorption. This study shows a new molecular approach to incorporate two chromophores in the dimeric dye possessing complementary absorption characteristics toward panchromatic absorption. The attenuated charge recombination at TiO 2 /Dye/redox couple interface in case of D 1 -D 1, owing to better passivation of TiO 2 surface, was elucidated through impedance analysis. The FT-IR spectrum of D 1 -D 1 adsorbed on TiO 2 film indicated both the carboxylic units were involved in chemisorption which makes strong coupling between dye and TiO 2 .